Hook all Fetch Api AJAX requests

Question

How would you hook all AJAX requests that use the Fetch Api? Previously we could do something like this to hook all XMLHttpRequest:

(function() {
    var origOpen = XMLHttpRequest.prototype.open;
    XMLHttpRequest.prototype.open = function() {
      console.log('request started!');
      this.addEventListener('load', function() {
          console.log('request completed!');
          console.log(this.readyState); //will always be 4 (ajax is completed successfully)
          console.log(this.responseText); //whatever the response was
      });
      origOpen.apply(this, arguments);
    };
  })();

Or better yet, if you wanted to add to the function above, how would you hook all Fetch Api and all XMLHttpRequest AJAX requests?

Answer 1

It is easy to do due to nature of promises:

var taperFunction = response => response;
var originalFetch = fetch;
fetch = (input, init) => originalFetch(input, init)
    .then(response => new Promise((resolve) => {
        resolve(taperFunction(response));
    }));

and a bit more complex(explicit) example:

var originalFetch = fetch;
fetch = (input, init) => {
    debugger; // do what ever you want with request or reject it immediately
    return originalFetch(input, init).then(response => {
        // it is not important to create new Promise in ctor, we can await existing one, then wrap result into new one
        return new Promise((resolve) => {
            response.clone() // we can invoke `json()` only once, but clone do the trick
                .json()
                .then(json => {
                    debugger; // do what ever you want with response, even `resolve(new Response(body, options));`
                    resolve(response);
                });
        });
    });
};

Answer 2

In fact, Fetch Api is supported by native browser and only has one interface: fetch. The constructor return one Promise, and you can't get the Request、Response when you want to return your Promise to rewrite the fetch's constructor.

The following code doesn't work well.

(function() {
    var oldFectch = fetch;
    fetch.consotructor = function() {
        return new Promise(resolve, reject) {
            // your hook code
        };
    };
})();

So,Does it mean we can't hook all Fetch Api ? NO!

Firstly, thanks the window.fetch polyfill.

Then, let's do something(edit fetch.js) and rock.

(function(self) {
    'use strict';

    // comment or delete the following code block
    // if (self.fetch) {
    //    return
    // }

    var support = {
        searchParams: 'URLSearchParams' in self,
        iterable: 'Symbol' in self && 'iterator' in Symbol,
        // ...

Last, hook every thing as your better!

self.fetch = function(input, init) {
  return new Promise(function(resolve, reject) {
    var request = new Request(input, init)
    var xhr = new XMLHttpRequest()

    // Here we go!
    // Now we get the XMLHttpRequest Object
    // Do what you want!

    xhr.onload = function() {
      var options = {
        status: xhr.status,
        statusText: xhr.statusText,
        headers: parseHeaders(xhr.getAllResponseHeaders() || '')
      }
      options.url = 'responseURL' in xhr ? xhr.responseURL : options.headers.get('X-Request-URL')
      var body = 'response' in xhr ? xhr.response : xhr.responseText
      resolve(new Response(body, options))
    }
    // ...